The invention relates to a bung closure for the gas-tight and liquid-tight closure of a bung opening of a container, in particular a steel drum for storing and for transporting in particular hazardous liquid charges.
Previously customary bung closures for tight-head steel drums are manufactured completely from metal and are generally known per se. A bung closure of this type is known, for example, from the document U.S. Pat. No. 7,287,662. The problem with said known bung closure is that, in certain applications, said bung closure has to be provided with a special, chemical-resistant phenol coating on the lower side coming into contact with the aggressive charge, including the external thread, to provide permanent protection against corrosion. However, this special paint coating can easily be abraded upon repeated unscrewing and screwing down of the bung closures, wherein, firstly, the paint abrasion then drops into the charge and contaminates the latter and, secondly, the abrasion points in the thread are coated too thinly or even no longer at all, and therefore corrosion forms there and the closure stoppers sometimes become badly jammed or very rusted in.
In addition, the screw threads of the bung closures have to have a highly accurate fit and low tolerances in order to be screwable in an absolutely tightly closed manner into the bung connectors with comparatively small tightening torques. There are only a few manufacturers of high-quality steel bung closures who divide up the corresponding market requirement at a high price level.
Bung closures, such as, in particular, tight-head steel drums, which are used, for example, in the chemical industry for storing and for transporting hazardous liquid charges (chemicals), customarily have to have passed an official design check. In said check, a container is subjected to different tests, such as, for example, an internal pressure test, a stacking test and a drop test, said tests also taking place at low temperatures. Said cold tests are carried out at −20° C., wherein the container is filled with a supercooled liquid which contains a high proportion of antifreezing agents. During said checking tests, the bung closures are inter alia also exposed with regard to the tightness thereof to high loadings.